Index of Refraction in the Critical Region

We investigate the dominant corrections in the critical region to previously derived expressions for the index of refraction $n$ of a nonpolar fluid. We show that the expression for $n$ has a form that can be used to test the assumption that the density-density correlation function is an asymptotically homogeneous function, and to measure the critical exponent $\eta$. We find that at the critical point both the real and imaginary parts of $n$ are finite under quite general assumptions, the change in $n$ owing to critical behavior remaining small. A simple but highly accurate approximate expression is given for the difference between real $n$ and its slowly varying background contribution $n_B$ in the critical region.